This invention relates generally to a rubber composition and more particularly concerns a rubber composition comprising carbon black and a second reinforcing filler consisting of kaolin, metakaolin, pyrophyllite, talc, coal, coke, or metal carbonate.
The precise formulation of any rubber compound depends on its intended end use. Different end uses require very specific mechanical properties. To a degree, the choice of filler and agent for coupling the rubber and filler determines the mechanical properties of the rubber compound. Because of the properties imparted to rubber compositions by carbon black, carbon black is the primary reinforcing filler almost universally used in certain types of rubber compositions, for example, those intended for use in tires for automobiles, trucks or other equipment. However, because of increases in the prices of petroleum products in general, and carbon black in particular, there has been great interest in replacing all or at least a significant part of the carbon black in rubber compositions with a less expensive material, in particular, one of the readily available non-carbon black fillers.
For example, Manual 3-S entitled "Sierra Mistron Vapor in Rubber Compounds," published in 1964 by Sierra Talc and Chemical Company (now Cyprus Mines Company), discloses that the replacement with talc of about 30 volume percent of the carbon black filler in a rubber composition affords a synergistic improvement in the tensile strength of the rubber composition.
Quaternary ammonium salts have also been disclosed for use with fillers or pigments. Technical Report 35-T entitled "Factors Affecting the Dispersion of Ultra-Fine Particle Size Mistron Talcs in Water and the Effects of Wetting and Dispersion Agents," published in 1956 by Sierra Talc & Clay Company (now Cyprus Mines Company) describes quaternary ammonium salts as effective dispersants for talc in water. Technical Report 48-T entitled "Mistrons in Hydrocarbon Solvents--Effects of Cationic Surfactants," published in 1958 by the former Sierra Talc & Clay Company describes certain quaternary ammonium salts as dispersants of talc in plastics, rubber and other polymeric dispersions. German Offenlegungsschrift 2,224,003 which issued in 1972 contains a very similar disclosure, E. K. Lotakova, G. A. Blokh, E. V. Tsipenyuk, I. G. Lebed, T. G. Blokh, F. D. Ovcharenko, N. G. Vasileo, A. D. Chugai and V. A. Mashchenko, "Properties of Rubbers Filled with Modified Kaolins," Kozh.-Obuvn. Prom-st. Vol. 18(6), pp. 45-7 (1976) discloses the reinforcement of rubber with kaolin modified with benzyldimethylalkylammonium chloride. N. S. Gilinskaya, G. A. Sankina, M. S. Skorobogatova, "Effect of the Chemical Nature of Dihydric Phenols and Quaternary Ammonium Salts on the Properties of Vulcanized Rubbers Made from the Rubber SKR-26," Kauch. Rezina, Vol. 3, pp. 10-12 (1980) discloses the vulcanization of flourorubber in the presence of quaternary ammonium salts.
Burke, U.S. Pat. Nos. 3,686,113; 3,689,452; and 3,840,382 disclose a rubber composition comprising a silica pigment, a quaternary ammonium salt and carbon black. Berg et al., U.S. Pat. Nos. 3,846,365; 3,922,240; and 3,998,778 disclose a method for making a powdered, flowable rubber composition comprising a solid filler wherein a quaternary ammonium salt is used as an emulsifier. Although In Berg et al. disclose in each patent that the filler is preferably carbon black, they state in each case that in place of or "in addition thereto, light-colored fillers can also be used, e.g., highly active silicic acid, kaolin, ground slate, etc." and in U.S. Pat. No. 3,922,240 that "[c]ombinations of carbon black with light-colored fillers, preferably silicic acid, are also possible." In the Berg methods, the rubber and filler composition is formed by pouring an aqueous emulsion of the filler, rubber elastomer, and the quaternary salt emulsifier into a sodium silicate solution, from which the granular rubber precipitates. Although Berg does not disclose the composition of the rubber granules, analysis of a polybutadiene rubber produced by a duplication of Example 1 of the '778 patent showed the rubber contained about 53 weight % of the silicate in the precipitating solution. Thus, all rubber compositions produced by Berg contain SiO.sub.2 as an unavoidable element. Moreover, Berg does not disclose the beneficial effect on modulus properties of a quaternary ammonium salt added as a coupling agent to rubber before vulcanization. Finally, the Berg et al. patents do not contain a disclosure of additional particular minerals that could be beneficially used in combination with carbon black and a quaternary ammonium salt in a rubber composition.
U.S. Pat. No. 4,032,501, "Dry Blendable Solution Rubber Powders and Process," Schulz, issued June 28, 1977, like Berg, discloses a method of producing a rubber powder. As an emulsifier, Schulz employs a "foam inhibiting surfactant" which is disclosed as either a nonionic polyether alcohol or a tetraalkyl ammonium salt having more than 25 total carbon atoms. Schulz, however, does not disclose use of clay as a second reinforcing filler, nor does he disclose the quaternary salts as a coupling agent.
Kealy et al., U.S. Pat. No. 3,627,723, "Filler Loaded Elastomer Compositions Having Improved Extrudability and Physical Properties", issued Dec. 14, 1971, disclose rubber compositions of an .alpha.-olefin/non-conjugated diene copolymer, a filler of carbon black or mineral or mixtures thereof, and a surfactant. Named as possible mineral fillers are kaolin, calcined kaolin, magnesium silicate, blance fixe, whiting, silica and talc. The surfactant can be one of three broad classes, anionic, nonionic, and cationic, and named are quaternary ammonium salts. The Kealy Examples show little difference in rubber properties due to a change in the surfactant. In contrast, only a quaternary ammonium salt exhibits improved rubber properties when used with a rubber such as natural rubber or styrene-butadiene rubber.